A manifest.json as well as a modlist or so is the required files to allow a modpack to download all the necessary mods and things required. If you export a mudpack for example you can tell what the necessary files are in there, if you allow configs to be required with the export too it will keep them in the .zip but the mods must be downloaded from curseforge hence why the manifest.json and modlist files are in there. I assume its so it can tell the mods it gets from curseforge compared to those not or just a download verification/process thing. I don't know I'm only guessing here, but it is the exportation/Twitch Launcher modpack downloading requirement the manifest.json I know since without it I couldn't upload a modpack to curseforge without one but had no problems since with the 'export' method compared to just a simple 'configs', 'mods' in a .zip folder isn't exceptable to curse unfortunately.
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Niche Mod Finder/Acknowledger/Tester/Player. Fan of Datapacks, Rift 1.13/1.13.2 Mods, Fabric 1.14 Mods and interesting underrated or new 1.12.2 and below versions of mods. :) Testing 1.13.2 Forge and Fabric 1.14 mods mostly though.
I sometimes create the odd modpack or adventure map once an idea comes together.